Heat transfer analysis of U-type deep borehole heat exchangers of geothermal energy

The paper describes a U-type deep borehole heat exchanger (DBHE) containing both two vertical boreholes and one horizontal boreholes for the application of medium-deep geothermal energy, the circulating fluid flows through every borehole in turn and therefore more underground heat can be absorbed. The boreholes are called as descending, horizontal and ascending boreholes, and the heat transfer tubes in the corresponding boreholes are called as descending, horizontal and ascending tubes, respectively. The backfill grout is set between borehole wall and the corresponding tube. The characteristics and the working principles of the U-type DBHE are discussed. The governing equations of heat transfer are established, and the numerical finite difference methods are employed to build the node equations for both the fluids in the heat transfer tubes and the underground medium, and the pursuit method is taken into consideration to obtain the temperatures used in the node equations. In addition, An actual project that uses U-type DBHE to heat the buildings was investigated. The research work can promote the further development and application of medium-deep geothermal heating technology. [Display omitted] •Describe the structural composition and working principle of borehole heat exchanger.•Explain the governing equations and the corresponding conditions.•Establish the node equations for fluid of tubes.•Establish the node equations for underground rock soil.•Investigate the change of important parameters of one actual project. This paper describes a U-type deep borehole heat exchanger (DBHE) containing both two vertical boreholes and one horizontal boreholes for the application of medium-deep geothermal energy, and presents the heat exchanger’s characteristics and the working principles. Compared with the single DBHE, the ability of the U-type DBHE to extract underground energy is better, and therefore a higher heating load of buildings can be assumed. First, the preconditions are explained, and the governing equations of heat transfer are established. Second, the numerical finite difference methods are employed to build the node equations for both the fluids in the heat transfer tubes and the underground medium. In addition, the pursuit method is taken into consideration to obtain the temperatures used in the node equations. Accordingly, the temperatures of the different locations in both the tubes and the underground medium can be acquired. Finally, one actual proj